1. Field of the Invention
The present invention relates to a toner density detecting apparatus to be used in an electrophotographic copying machine and an electrostatic recording apparatus.
2. Description of the Prior Art
In general, in the transfer type electrophotographic copying apparatus and the electrostatic recording apparatus, a developer containing a magnetic carrier material and an electrically insulative coloring toner is used for converting an electrostatic latent image produced on a light- or photo-sensitive medium into a visible image. The toner material is electrostatically attracted onto the electrostatic latent image corresponding to an original to be copied. Then the latent image which has become a visible image (toner image) is transferred onto a copying sheet (plain paper) and thermally fused thereon. In order to attain a constant image density in the replica as produced in the copying or recording system of the above type, it is necessary to measure the density or concentration of the toner contained in the developer as the toner is consumed thereby to supplement the toner so as to maintain the toner concentration or density substantially constant. Heretofore, various toner density detecting systems have been proposed.
As one of the toner density detecting methods, it is known to detect a variation in permeability of the developer as a variation in inductance of a coil since the permeability of the developer varies as a function of toner concentration (refer to Japanese Patent Publication No. 8280/1971, for example.). Since the development of a latent image by using a developer is usually effected when a photo-sensitive medium carrying an electrostatic latent image thereon is being moved together with the developer in contact with the latent image, it is necessary to produce a steady and uniform flow of the developer through the coil which is to be detected to measure the concentration or density of the toner in accordance with the method described above. Apparatus implemented on this principle are disclosed in Japanese Laid-Open Patent Exposure No. 123242/1977. Typical examples of such prior art toner density detecting systems are schematically illustrated in FIGS. 1 to 3 and 5 of the accompanying drawings. In the Figures, identical reference numerals denote the same or similar parts.
Referring first, to FIG. 1, a rotatable sleeve 2 having an inner magnetic assembly or magnet assembly including a plurality of permanent magnets 1 disposed therein is located so as to transport the developer 4 from a developer container 3 to a region of a photosensitive medium or an electrostatic recording medium 5 in which an electrostatic latent image 6 to be developed is produced. Thus, the developer is caused to contact the latent image to convert it into a corresponding toner image. After development, the residual developer is scraped from the sleeve 2 by means of a scraper 7, a fraction of which is introduced into a passage defined by a coil bobbin 8 on which a coil 9 is wound. Passage of the developer 4 through the bobbin 8 will bring about a variation in the inductance of the coil 9, which variation is measured by an external detecting circuit (not shown) to thereby control the toner density. Reference numeral 10 denotes a fluffing restricting plate (thickness regulating plate) for regulating the thickness of the flowing developer layer. In the case of the toner density detecting system illustrated in FIG. 1, special means having an expensive and complicated structure is required for attaining a steady and uniform flow of the developer through the coil bobbin 8, which necessarily involves a large and complex developing device. Further, because the measurement of the inductance of the coil 9 must be delayed until the bobbin 8 has been filled with the developer and the developer has fallen to a position at which the coil 9 is disposed, a time lag is involved in the measurement, accompanied by a disadvantageously deteriorated measurement accuracy.
In an attempt to avoid the shortcomings described above, there have been proposed the apparatus shown in FIGS. 2 and 3, in which the inherent flow of the developer within the developing device is used for the detection of the toner density without resorting to the use of a flow deflecting means such as the scraper 7 shown in FIG. 1. Referring to FIGS. 2 and 3, a flat coil 9' is employed which undergoes variation in the inductance thereof. In the case of the arrangement shown in FIG. 2, the flat coil 9' is mounted on the thickness regulating plate 10, while in the case of the apparatus shown in FIG. 3 the flat coil 9' is mounted on the bottom plate of the developer container 3 in such a position as not to obstruct the flow of the developer. By the way, the flat coil itself has been hitherto known and may be composed of a planar winding embedded or wrapped in a rectangular flat plate 11 formed of a plastic material as is shown in FIG. 4. Alternatively, a printed coil may of course be used. The arrangements shown in FIGS. 2 and 3 are certainly advantageous in that the provision of the coil 9' does not interfere with the flow of the developer. However, because only one face of the flat coil is used for the detection of the toner density, it is difficult to attain a sufficiently high sensitivity in the detection system. Moreover, in the case of the apparatus shown in FIG. 2, the function of the plate 10 for restricting the fluffing of the developer might be adversely influenced due to the provision of the coil. Furthermore, the permanent magnets (indicated by the broken or imaginary lines) constituting the magnet roll assembly 1 are likely to interfere with the measurement, making it difficult to detect the toner density with a desired high accuracy. On the other hand, in the case of the arrangement shown In FIG. 3, remarkable turbulences will occur at the region located below the rotating sleeve 2 due to various flows of the developer such as the feeding flow toward the rotating sleeve, dropping of the developer from the fluffing or thickness regulating plate 10 and the flow caused under the action of the permanent magnets of the rotating sleeve 2, whereby a significant degradation in S/N ratio will result. Additionally, a preset relationship between the toner density and the inductance of the coil may undergo irregular variations, whereby a slight change in the toner content may give rise to a significant deviation in the measured density.
As another approach for overcoming the drawbacks described above, there also has been proposed an arrangement shown in FIG. 5 in which a flow rectifying plate 12 is disposed in opposition and parallel to the scraper plate 7 with flat coils 9', such as the one shown in FIG. 4, being mounted on the lower surface of the scraper 7 and on the upper surface of the flow rectifying plate 12, respectively, whereby a fraction of the developer is caused to flow between the scraper 7 and the rectifying plate 12. By virtue of the fact that two coils are used, the apparatus shown in FIG. 5 can enjoy a high sensitivity. However, this structure suffers from disadvantages in that the developing device becomes complicated in structure and expensive in manufacturing costs.